Tech Tidbit: Why an “Obsolete” Pushrod Engine Design is Better Than Modern Overhead Cams – Dan Carney @DesignNews

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The roots of today’s pushrod engines stretch back to the post-war years.

Contemplating the advantages of the pushrod-actuated overhead valve engine design in what appears to be the twilight of internal combustion might sound as anachronistic as a quick look at the value of coal-fired steam locomotives.

But design advances in these engines, particularly the larger-displacement V8 variety beloved by American drivers and automakers alike, mean that these engines boast advantages that seem certain to keep them relevant until the very end of combustion power.

Let’s start with some definitions. Overhead valve pushrod engines marked an advance over their predecessors, the flathead engines whose valves are located in the engine block alongside the cylinder. The Ford flathead V8 and the classic Briggs & Stratton power equipment engine are well-known examples.

These engines are compact, inexpensive to manufacture, and woefully inefficient because of poor airflow, combustion, and thermal characteristics. Flathead engines’ combustion chamber is wide and flat, covering the piston top and the valves, making combustion very ineffective. Consider it the opposite of the concentrated, semi-circular combustion chamber shape of the Hemi engines we discussed previously.

Additionally, the airflow into and out of the combustion chamber is indirect, as the air must make hard 90-degree turns. And the intake and exhaust ports are located adjacently on the same side of the cylinder, transferring exhaust heat to the intake charge, reducing its density and resulting power.

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Categories: Engine, V8

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